The invention relates to a system and method for optimizing the charging of batteries in electric and hybrid vehicles.
Electric vehicles have an electric motor as a power source that uses batteries or fuel cells as the source of energy. Hybrid vehicles generally have two different power sources to drive the vehicle, usually one being an internal combustion engine and the other an electric motor that is powered by an energy source, such as batteries. The batteries are of the rechargeable type. Other types of energy sources such as super-capacitors also can be used. Both kinds of vehicles also are usually equipped with a regenerative system which converts vehicle kinetic energy into electrical energy to recharge the energy source, here considered to be one or more batteries. For example, as the vehicle undergoes braking, the braking force drives the electric motor of the vehicle operated as a generator, or drives a separate generator, that is used to generate electrical energy (current) to recharge the batteries. The electric energy produced by the regenerative system is stored in the vehicle batteries and is used to power the vehicle electric motor when needed.
When batteries are used in electric or hybrid vehicles, they are generally maintained in a state of charge (SOC) range at which the battery internal resistance (IR) is minimal, especially in a charging condition. This is done for the purpose of preventing excessive heating and to have high efficiency of charging, meaning that most of the energy goes to charging and very little is wasted in heating due to the high impedance of the battery, from the regenerative braking system. For example, for lead-acid batteries, these often being used in electric and hybrid vehicles, the batteries are kept at a relatively low SOC level of around 60%-65%. However, batteries tend to degrade faster under the condition of prolonged time in undercharged (low SOC) condition. For example, lead acid batteries tend to become sulfated and thereby have a shortened battery life.
Normally, it has been believed in the state of the art that when an electric or hybrid vehicle battery is above 70% SOC that the charge is accompanied by an undesirable overcharge gas evolution reaction. This is particularly true for lead acid batteries. I have determined that this holds true only when the battery is charged after it has been allowed to reach an equilibrium state. A battery reaches an equilibrium state after it is allowed to rest (no charge or discharge) for a period of time, for example, about 2-3 hours in a lead acid battery. By looking at the battery in situ current voltage characteristics, it can be determined if the battery is at equilibrium or not. This is done by determining if a current is present, whether positive or negative, and if it is, the battery is not in equilibrium. Similarly, if the voltage is above the upper limit or below a lower limit it is not in equilibrium.
In electric and hybrid vehicles, the regenerative energy is dumped (charged) into the battery when it is not at equilibrium. Generally, the battery keeps discharging as it is used to power the electric motor until the instant when the brakes are applied and at this time the regenerative energy is dumped into the battery. Typically, in the present state of the art, even though a battery of an electric or hybrid vehicle is charged before it reaches a state of equilibrium while in the vehicle by the regenerative system, the SOC value is still held at about 65%. That is, using present technology, the amount of the regenerative energy supplied to charge the battery is controlled as a function of battery SOC so that the battery SOC does not exceed 65%.
Accordingly, a need exists to control the charge of a battery in an electric or hybrid vehicle to place and maintain it at a higher level of SOC.
I have determined that a battery can be charged at very high efficiency if the battery is charged immediately after a pulse type discharge or a continuous discharge. In an electric or hybrid vehicle, the continuous discharge would use the batteries as a source over a Is period of time to power the vehicle electric motor. The pulse would be a short burst of use of the vehicle electric motor. It has been found that when a battery is discharged by current pulses or continuous current, it can be immediately charged up to about 80% of the energy taken out during the discharge. This can be accomplished even if the battery SOC is above 80% when the discharge is stopped. It also has been found that a battery can be charged to a higher SOC than the 65% value that is currently used in electric and hybrid vehicles.
In accordance with my invention, the regenerative system is operated to immediately charge the battery upon discharge being terminated. The charging is carried out to the maximum extent possible, the charge current limitation being predominantly determined by the circuit characteristics of the battery charging system, such as the current carrying capacity of the wires and other components. Hardware control elements in the battery charging system are provided to prevent the charging current from rising above this safe level. Normally, charging current will be limited automatically to a smaller value than the safe limit when the battery voltage is controlled to a desired level.
In any system, as a battery is charged, its SOC increases over time. Thus, a battery can be charged to a high SOC value merely by continuing the charging time. In the present invention, the dumping of the regenerative energy into the battery for its charge is controlled as a factor of battery charge current and battery voltage limitations instead of the SOC, which is used in the present systems to achieve and maintain a higher level than the 65% SOC that is currently used.
In a preferred embodiment of the invention, a higher SOC is obtained. As is known, at a certain point during charge, a battery will start to produce gas. I have determined that there is a relationship between the current and voltage of the battery during charging and the time at which the gas point is reached. In accordance with the preferred embodiment of my invention, the charge voltage is monitored and is limited based on the battery gas point characteristics, such as Igas and Vgas. At the gas point, at least for a lead-acid battery, the SOC will be higher than 65%, often in the range of 80%-90%, depending on the battery construction. The charging is limited or terminated before the gas point is reached. The gas point parameters may be determined from battery parameters other than the in situ current-voltage characteristics. For example, the battery voltage during charging or rate of change of battery internal impedance may be used to detect the gas point.
By following guidelines listed above, hybrid and electric vehicle batteries can be charged at greater efficiency when not in a state of equilibrium and also operated at a higher level of SOC, for example, around 80% SOC, and perhaps even up to 90% SOC. This contrasts with the present 60%-65% SOC for present day hybrid and electric vehicle batteries. This higher SOC level of operation results in longer battery life and better fuel efficiency in the case of hybrid vehicles.
It is an object of the invention to provide a method and apparatus for controlling the charging of batteries in electric and hybrid vehicles to an optimal SOC value.
Another object is to provide a method and system for charging batteries in electric and hybrid vehicles in which the batteries are charged at a high current level after a continuous discharge or pulse discharge under dynamic operating conditions to a high level of SOC as determined by the battery voltage level during charge.
Yet another object is to provide a method and system for optimizing the charging of batteries in electric or hybrid vehicles in which charging is controlled to bring the batteries to a relatively high level SOC.
Still a further object is to provide a method and system for charging the batteries in electric or hybrid vehicles to a relatively high level SOC without exceeding the gas point of the batteries.